A. Technical Field
The present invention relates to a dimmable light emitting diode (hereinafter, “LED”) illumination system, and more particularly, to systems, devices and methods of driving a LED module by a current generator that is powered and controlled by a regulated dc voltage associated with a brightness level. Such a dimmable LED illumination system is compatible with both a high-voltage ac signal coupled from any wall outlet and a low-voltage ac signal provided by an electronic transformer.
B. Background of the Invention
Semiconductor-based solid-state lighting (SSL), until recently associated mainly with simple indicator lamps in electronics and toys, has become as bright and more efficient than other lighting technologies, such as halogen lamps. In particular, the enormous technology improvements have been achieved on light emitting diodes (LEDs) over the past years. LEDs have been available for various wavelengths, and suitable for white illumination. Lifetime of LEDs is also extended to more than 100 thousand hours, and can work at input powers up to many watts.
When compared to halogen lamps, LEDs are relatively smaller, and have a longer operating life. Moreover, LEDs also require significantly less amount of power to operate than a typical halogen lamp, e.g., MR16. For example, a halogen lamp may operate within a range of 20-50 Watts, while an LED at about 5-10 Watts is sufficient to provide a similar level of brightness.
The halogen lamp is primarily resistive, and a halogen illumination system may be coupled to an electronic transformer in MR16 applications, requiring a minimum power of 20 watts for proper operation. Although customers would like to replace the halogen lamp with LEDs, the original illumination system may not be compatible to the LEDs or properly used to couple the LEDs with the electronic transformers. A single LED lamp consumes a power in the range of 5-10 watts, and when it is loaded to the conventional halogen illumination system, it is inadequate to keep the electronic transformer operational. This problem is addressed by a method disclosed in a previously-filed provisional application 61/526,857. A transformer load compensation circuitry is attached at an output of the electronic transformer to rapidly ramp up a transformer current and a LED driver current.
The dimming control method used to control a resistive halogen lamp is not applicable to control the LEDs that are diodes. FIG. 1 illustrates an exemplary dimmable illumination system 100 based on a LED lamp which operates at a low frequency ac source. In this example, an ac signal is first rectified in a diode bridge 104 and subsequently used by a LED driver 106 to generate a LED current ILED that drives a LED module 108. A dimmer 102 is placed in the path of the ac signal such that duty cycles of the ac signal are offset at leading and trailing edges according to a desired brightness level.
The LED driver 106 comprises two power processing stages, i.e., a boost power processing stage 110 followed by a current regulating stage 112. A capacitor C1 is an energy storage electrolytic capacitor that is applied to reduce voltage ripples for the voltage between two power processing stages. Most applications require power factor correction in the boost power processing stage 110 so that the input current drawn by the LED driver 106 is in phase with the rectified ac signal at the input of the LED driver. In particular, to limit a voltage rating of a switching transistor used in the boost stage 110, the output voltage of the boost stage 110 is regulated to a substantially constant dc voltage independent of the rectified ac signal or the output LED current.
The LED current is determined according to the rectified ac signal. The LED driver further comprises a LED current programming stage 114 that is coupled to the diode bridge 104 and generates a LED current control. The level or the duty cycle of the LED current is adjusted in the current generating stage 112 according to the LED current control, when the dimmable illumination system 100 adopts dc dimming or pulse width modulation (PWM) of the LED current, respectively. The rectified ac signal is associated with the desired brightness level, and the LED current is accordingly adjusted to enable this brightness level in the LED module 108.
Although it functions properly when the ac signal is directly provided from the wall output, the dimmable illumination system 100 fails when an electronic transformer is applied to convert the ac signal to a lower-voltage ac signal. The electronic transformer may not start up due to a reduced load at the output of the diode bridge 104. This reduced load results not only from the LED module 108 but also from a negative feedback that is formed in the LED driver 106 largely due to the use of the LED current programming stage 114. A need exists to improve the LED current programming method in such a dimmable illumination system so that the electronic transformer may be applied as well.